The present invention relates to processes for producing composite substrates by joining metals to ceramics. The invention also relates to brazing materials that can effectively be used in joining metals to ceramics.
Several methods are known in the art of joining metal (Cu) plates to ceramic (aluminum nitride or AlN) substrates and two typical examples are the active metal brazing process described in Unexamined Published Japanese Patent Application (kokai) No. 166165/1985 entitled "A Method of Joining Nitride-Base Ceramics to Metals" and the DBC process in which a copper plate is directly joined to a modified surface of an aluminum nitride substrate (as typically described in Unexamined Published Japanese Patent Application No. 163093/1981).
The active metal brazing process provides a higher strength of joint than the DBC process and the resulting joint has desired characteristics such as high durability to repeated heat cycles. Hence, the active metal brazing process currently finds extensive use in the joining of copper plates to non-oxide-base ceramic substrates, for example, nitride ceramic substrates.
Two brazing materials are commercially used in the practice of the active metal brazing process; one of them is composed of Ag, Cu and an active metal selected from among Ti, Zr and Hf (as described in Unexamined Published Japanese Patent Application No. 166165/1985) and the other is an active metal paste material composed of Ag, Cu and titanium hydride (as described in Unexamined Published Japanese Patent Application No. 101153/1991).
Producing circuit substrates by joining a copper plate to both sides of an aluminum nitride substrate with the aid of these brazing materials has already been commercialized; however, most recent circuit substrates are required to perform on larger electrical power and to meet this requirement, it has been desired to develop circuit substrates that are not only provided with good heat dissipating and electrical insulating properties but which also exhibit higher strength and thermal impact resistance